It has become increasingly important to understand the relationship between dietary intake and blood chemistry and hematological profiles in pinniped populations, especially where nutritional stress has been implicated in population declines. Blood chemistry and hematology reference ranges have been commonly reported for both free-ranging and captive harbor seals, but seasonal and dietary fluctuations are limited to only a few studies. We quantified changes in plasma metabolites and hematology values in captive harbor seals on different diets over two years. Of 29 blood chemistry and hematology variables examined in captive harbor seals, ALT, AST, GGT, creatinine, and BUN:creatinine were influenced by diet. BUN, Na+, Cl−, Hct, and Hb were influenced by season. Diet did not affect hematology variables. While these data were collected on captive animals, we assume seasonal and dietary influences would be comparable in free-ranging seals. According to this study, controlling for season during sampling becomes important when assessing blood chemistry values as an index of foraging status or health.